Fasted workouts, how long is too long?

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  • #8131

    Hi all. Thank you for “Training for the New Alpinism” and the ressources on the Uphill athlete website. They were very usefull to me last year to achieve my goals: a couple of short trails (20k), and passing the entry exam for the French International Mountain leader award (Accompagnateur en Moyenne Montagne).

    My question is in the title: Fasted workouts, how long is too long?

    During my training last year, I gradually switched my outings from afternoon to early morning, fasted workouts. After a while, I would easily run 10-12k sometimes a bit more, fasted (between 1hr and 1hr1/2) several times a week. I would make sure I had some food for longer outings, whether hiking or running

    I hear some people say we shouldn’t go over 1 hour without food. But I don’t recall reading anything about that in the book or on the website. So, as I start building up the mileage again, how long can I go without food? Is there a limit to how long we should go?

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    Anonymous on #8134

    @keloumat: You’re on the right track.

    The duration of fasted workouts is very personal. It varies for everyone, but it sounds to me like you’re progressing in the right direction with a good strategy.

    I don’t think there’s any reason to limit the duration of fasted workouts. For me, I usually eat if I start feeling “bonky”. When that is will vary per person.

    Anonymous on #8158

    I agree with SS. There is now reason you can not run several hours fasted if you become fat adapted. I routinely do easy runs and ski sessions (both XC and Skimo) of 2-3 hours fasted and have no problems. I know many top athletes that can also do several hours fasted without issue. If the intensity is high then you all need to eat of course but for most people training by running 2 hours will be necessity be low intensity where fat should be the main fuel. If you want to increase fat adaptation then gradually increase the length of these runs until they are comfortable for you.
    When you go with out eating in a longer workout you must expect that your recover will be much slower than if you eat during the workout. If you are trying to build up many miles/km or to do some high intensity then you need to eat more often and maybe not do any fasted workouts. But if another time you want to work mainly on improving your fat adaptation then go longer and be prepared to feel not so good for the next 2 days of training where you will need to go easy.


    keloumat on #8162

    Thank you for your replies. I’ll continue not to worry too much about doing fasted workouts for longer and gradually build up to go further. I suppose it is always safe to keep somethig to eat in case I start feeling ‘bonky’ at some point. What should I carry though? A gel or two, cereal bar, nuts?

    Anonymous on #8164

    Food is pretty personal, so I would go with whatever you like.

    One thing to consider, however, is to favor fat-dominant food over carb-dominant. This is only anecdotal, but I find that if I eat fat-dominant food (nuts, cheese, sausage), then I don’t need to take as much food with me. If I eat more glycemic food (i.e. food that is more carb-dominant), then I get hungry faster.

    The exception to this is for racing, where more carbs are required.

    keloumat on #8179

    Just back from a nice fasted workout, 12K, 1h22 minutes on hilly ground at a nice pace well within my aerobic zone. I’ll gradually add more fasted workouts in the coming weeks in order to do most of them fasted, except for the longer ones, like what I did last spring.

    Concerning food, I think you’re right Scott. When on a long day out hiking, I fond that if my breakfast is rich in proteins and fat dominant foods (usually porridge with semi-skimmed milk and chia seeds and a bit of sugar), I’ll take nuts in my pockets and don’t need to eat much else for most of the day appart from snacking on those. I’ll eat my sandwiches back at the car.

    Anonymous on #8181

    Just my .02, I do 3-4h fasted runs, once per week. Under guidance of Rebecca Dent, I wake up, make a cup of black coffee and nurse over the next 30m while I lounge around, then get my stuff together and run. I keep these entirely moderate Z1 (~30km in 3.5h) so I don’t creep into Z3 on accident and bonk. On these runs I bring 1L water in a small pack and two Gu shots, just in case I do start to lose it. My girlfriend bonked once around the 2:45 mark and a Gu had her right as rain within 5 minutes. I bring these instead of real food since bonking away from home is not ideal and it works fast. As soon as I get home I make a recovery shake with one scoop protein, peanut butter, and a scoop of oats. Sometimes I will toss in half of a banana and some flax seeds or something. After that I shower up and get real food as soon as possible.


    Anonymous on #8197

    I second @adamsc’s idea to include coffee for fasted sessions, but with a caveat.

    Some of my favorite workouts are fasted runs first thing in the morning after a strong, black coffee. However, I react quite strongly to caffeine, and it seems to shorten the amount of time that I can run fasted. Basically, if I use caffeine I get hungry sooner.

    If you’re going to try it, then follow Adam’s advice and have a couple emergency gels handy. And maybe start with shorter sessions to experiment.

    keloumat on #8310

    Thank you all for your reply. I hadn’t mentioned coffee because i was taking it for granted, but yes, I usually have black coffee or black tea, without sugar of course before going on a fasted outing.

    cnikirk on #9202

    This is something that confuses me. On fasted workouts I try to avoid gel shots even if I bonk. Won’t taking carbs on fasted workout reset your body to think it needs carbs? Maybe I’m doing it wrong. It is horrible to bonk and push through it, but I’ve done it. Thanks.

    Anonymous on #9213

    @cnkirk: I think the two things to keep in mind are: progress gradually and be safe. A mistake that I’ve made many, many, many times is to decide on a workout or goal and gut it out to the finish even though it becomes counter-productive. That approach seems very common among alpine climbers in particular (of which I was one).

    I mention that attitude because it’s not as effective as a more mellow, gradual progression. If you do a fasted workout and start bonking, note the duration that you started feeling that way. Next, maybe push it for 15-30′ and then eat something. On your next outing, note when you’re feeling bonky and do the same thing. I’ve found that, gradually, that bonky feeling will happen later and later.

    I mention safety as well because these workouts shouldn’t be done if there are consequences to bad decisions. Bonking will affect how clearly you can think, so you want to experiment with it in mellow terrain.

    I hope that helps.

    cnikirk on #9219

    That’s excellent thank you for clearing that up. I felt like I was destroying my progress if I gave in. By the way I haven’t given in yet 😉

    Anonymous on #9235

    I’ve become a big fan of giving in.

    Starting from an alpine climbing background, when I came to structured training, I drove myself into the ground many, many times. In a chronic training environment, that approach doesn’t work. (Although yes, it does work well in an unstructured training regime with random and frequent periods of complete rest.)

    I’ve made much better progress since I clued into when enough is enough.

    “Do as little as necessary, not as much as possible.”
    ~ Henk Kraaijenhof

    For more of the same advice, check out this recent post from Steve Magness:

    hafjell on #9423

    Is it ok to go out on a fasted run without coffee? I don’t/won’t drink it, but would consider an alternative if necessary.
    I did my first fasted run this morning, one hour, and felt great. No coffee but did drink water before and during.

    Thrusthamster on #9428


    The energy systems the body uses aren’t like a switch where one is turned on and the other is turned off. They’re working at the same time but at different levels for different intensities. I don’t have a citation for it, but it might be the case that by having some quick carbohydrates the energy system that metabolizes carbohydrates recovers a bit and can take up the slack for a while and relieve the other systems, making you feel a bit better? It’s just speculation on my part.

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